The present invention relates generally to a taking zoom lens system for lens shutter cameras, which comprises two units, the first unit having a negative refracting power and the second unit having a positive refracting power, and more particularly to a two-unit zoom lens system designed for waterproof purposes.
So far, many positive-unit preceding type zoom lens systems, represented by a two-unit or positive/negative-unit zoom lens system and a three-unit or positive/positive/negative-unit zoom lens system, have been proposed and practically used as zoom lens systems for lens shutter cameras. However, the positive-unit preceding type zoom lens systems are unpreferred, because the amount of movement of the first units in association with zooming is so increased that the mechanical structure for water is complicated.
In order to achieve waterproofness with a simple structure, it is thus desired to eliminate, or at least reduce, the amount of movement of the first unit in association with zooming. Negative-unit preceding type zoom lens systems have been known to have such a feature. In particular, a two-unit or negative/positive-unit type zoom lens system is often practically used as the standard zoom lens for a single-lens reflex camera's interchangeable lens. However, the single-lens reflex camera lens is not suitable for lens shutter cameras without making some modification thereto, because the back focus is long and hence the total lens length is increased.
As known so far in the art, reducing the total lens length may be achieved by constructing the unit on the image-taking side in positive/negative telephoto type. For instance, Japanese Provisional Patent Publication No. 62-50718 sets forth an example of applying this concept to a two-unit or negative/positive-unit zoom lens system.
According to Japanese Provisional Patent Publication No. 62-50718, reducing the total lens length is achieved by constructing the second unit having a positive refracting power of positive and negative lens components.
However, the zoom lens system set forth in Japanese Provisional Patent Publication No. 62-50718 has a cost problem, because it comprises as many as 9 lens components plus two additional aspherical surfaces.